Chemokines and their receptor are best known for their role in immune surveillance, where they control the migration and activation of leukocytes to resolve physiological abnormalities such as infection and cancer. However, inappropriate regulation of these proteins is associated with an extraordinary number of pathologies including inflammatory disease, atherosclerosis, cancer and AIDS. Thus there is significant interest in understanding how they function in order to develop drugs to block their activities. In recent years, it has become clear that chemokine-induced cell migration is a complex multistep process involving many different interactions, not only of chemokines with chemokine receptors, but also with glycosaminoglycans (GAGs) and with each other through homo- and hetero-oligomerization. GAGs are structurally diverse linear carbohydrates that are often attached to membrane bound protein anchors as proteoglycans. These interactions are involved in locally sequestering chemokines on cell surfaces, preventing diffusion so that chemokines provide directional cues for migrating cells. Other roles for chemokine:GAG interactions include transport across cells, regulation of oligomerization with as yet poorly characterized signaling consequences, and regulation of receptor binding and activation. Since GAG structures are highly diverse, interactions with chemokines may significantly contribute to the specificity and fine tuning of cell migration, well beyond the chemokine:receptor interaction. However, at present, there is little structural information about chemokine:GAG complexes and their precise functional consequences. An overarching hypothesis of this proposal, is that structural plasticity through oligomerization of chemokines on GAGs, or GAG-induced folding of unstructured domains, are mechanisms of functional specificity/diversity, and that through changes in oligomerization or unfolded state structure, a given chemokine may recognize different GAGs. A second hypothesis is that some chemokines dynamically change their structures through homo- and hetero-oligomerization which may control other steps in cell migration in addition to immobilization on GAGs. Aim 1 involves structural studies of chemokine:GAG complexes to (i) identify GAGs that preferentially recognize specific chemokines and determine the level of specificity; (ii) characterize binding affinities and interaction mechanisms (e.g. oligomerization or folding of unstructured domains); and (iii) determine trans-scale structural information of complexes from small oligomers through larger oligomeric assemblies on flexible GAGs. To complement these studies, in Aim 2, structural information will be correlated with function to probe how dynamic changes in chemokine oligomerization and GAG interactions coordinate discrete sequential steps of cell migration: (i) transcytosis and cell surface presentation on GAGs, (ii) leukocyte arrest (iii) transmigration, and (iv) activation of cellular defense responses. Together these studies will provide novel insight into the regulation of chemokine function through GAG- binding and oligomerization, where structural plasticity is likely to play a significant role.